Method of forming a rewettable asymmetric membrane

ABSTRACT

The present disclosure provides rewettable asymmetric membranes and methods of forming rewettable asymmetric membranes. More specifically, methods are provided for forming rewettable asymmetric membranes having a copolymer and a polymerized material retained within the porous substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US 2009/043690, filed May 13, 2009, which claims priority toProvisional Application Ser. No. 61/077,077, filed Jun. 30, 2008, thedisclosures of which are incorporated by reference in their entiretyherein.

FIELD

The present disclosure relates to a method of forming a rewettableasymmetric membrane.

BACKGROUND

Membranes can be used in separation processes where certain species areretained and other species are allowed to pass through the membrane.Some membrane applications include, for example, use in food andbeverage, pharmaceutical, medical, automotive, electronic, chemical,biotechnology, and dairy industries.

Rewettable asymmetric membranes have been described.

SUMMARY

The present disclosure provides rewettable asymmetric membranes andmethods of forming rewettable asymmetric membranes. More specifically,methods are provided for forming rewettable asymmetric membranes havinga copolymer and a polymerized material retained within the poroussubstrate.

In one aspect, a method of forming a rewettable asymmetric membrane isprovided, the method comprising:

-   -   providing a porous substrate having a first major surface,        interstitial pores, and a second major surface;    -   applying a polymerizable composition to the first major surface        of the porous substrate to provide a coated porous substrate,        the polymerizable composition comprising        -   i) at least one polymerizable species;        -   ii) at least one copolymer comprising hydrophilic and            hydrophobic groups; and        -   iii) at least one photoinitiator; and    -   exposing the coated porous substrate to ultraviolet radiation to        polymerize the polymerizable composition and provide the        rewettable asymmetric membrane, the membrane comprising a        gradient of polymerized material extending from the first major        surface to the second major surface with copolymer within        portions of the interstitial pores unoccupied by the polymerized        material, and wherein the second major surface is substantially        free of the polymerized material.

In another aspect, a rewettable asymmetric membrane is provided. Therewettable asymmetric membrane comprises a porous substrate having apolymerized material and a copolymer retained within the poroussubstrate. The rewettable asymmetric membrane has a gradient ofpolymerized material extending from a first major surface to a secondmajor surface such that the copolymer collects on a portion of theinterstitial pores unoccupied by the polymerized material and the secondmajor surface is substantially free of the polymerized material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a coated porous substrateirradiated with an ultraviolet radiation source forming a rewettableasymmetric membrane.

DETAILED DESCRIPTION

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.8, 4, and 5).

As included in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Thus, for example, reference to acomposition containing “a compound” includes a mixture of two or morecompounds. As used in this specification and appended claims, the term“or” is generally employed in its sense including “and/or” unless thecontent clearly dictates otherwise.

Unless otherwise indicated, all numbers expressing quantities oringredients, measurement of properties and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about.”

In the method of the invention, a rewettable asymmetric membrane isprovided. Ultraviolet radiation is utilized for initiating apolymerization reaction in polymerizable compositions to thereby providea polymerized material that is retained within the pores of a poroussubstrate. The polymerized material is retained in a concentrationgradient extending at least partially through the thickness of thesubstrate with copolymer collected on inner portions of the poresunoccupied by the polymerized material.

A number of methods can be suitable for forming a gradient ofpolymerized material extending at least partially through the thicknessof the porous substrate. One method involves coating one side of thesubstrate with polymerizable composition and thereafter exposing theporous substrate and the polymerizable composition to ultravioletradiation such that there is a gradient intensity of light penetratingthrough at least a portion of the thickness of the substrate. A lightabsorbing material (e.g., a photoblocker) might be added to thepolymerizable composition wherein the specific photoblocker is selectedon the basis of the specific wavelength of radiation to be blocked, theextinction coefficient of the light absorbing material at the prescribedwavelength, and the absence of adverse photoreactions or adverseinvolvement in the polymerization reaction. One example of aphotoblocker for use with an ultraviolet radiation source having a peakemission wavelength of 350 nm is2,2′-dihydroxy-4,4′-dimethoxybenzophenone.

In some embodiments, polymerizable composition comprises a polymerizablespecies, photoinitiator and photoblocker.

In some embodiments, a source of ultraviolet radiation can be selectedfor delivering radiation to a coated porous substrate such that theirradiance delivered to the first major surface of the substrate isgreater than the irradiance delivered at the second major surface. Theirradiance can decrease as the radiation travels and is absorbedprogressing through the thickness of the coated porous substrate. Duringexposure to the ultraviolet radiation source, polymerizable compositionlocated at or near the first major surface receives a greater irradiancethan polymerizable composition at or near the second major surface.

In one embodiment, the ultraviolet radiation source has a peak emissionwavelength less than 340 nm.

The method of the present disclosure provides for a continuous processfor forming rewettable asymmetrical membranes having a higher fluxrelative to rewettable symmetrical membranes of the same composition.The term “asymmetric” refers to a membrane in which the pore size and/orstructure are not the same from one side of the membrane to the otherside. Typically, the pores of the rewettable asymmetric membranes arepartially filled (e.g., gel-filled) with polymerized material and acopolymer having both hydrophilic and hydrophobic groups. In themanufacture of the rewettable asymmetric membrane, a porous substrate iscoated with polymerizable composition such that a copolymer andpolymerizable species in the composition collect on the major surfacesand within the thickness of the porous substrate. The polymerizablecomposition can penetrate or saturate the thickness of the poroussubstrate, wetting the interstitial pores within. Irradiating one sideof the coated porous substrate with an ultraviolet radiation sourceunder an inert environment (e.g., substantially free of oxygen) resultsin a membrane having an asymmetrical distribution of polymerizedmaterial mixed with a copolymer and retained within the poroussubstrate. The resulting concentration of polymerized material isasymmetrically distributed through the thickness of the membrane with ahigher concentration of polymerized material at or ear one side (e.g.,at or near a first major surface) of the membrane and a lowerconcentration at or near the other side (e.g., at or near the secondmajor surface) of the membrane. Copolymer collects on surfaces withinthe pores that are not occupied by the polymerized material. Theremainder of the interstitial pores can be coated with the copolymersuch that the second major surface is substantially free of thepolymerized material. The process can be accomplished without theaddition of 1) high concentrations of photoinitiator and/or 2)photoblockers, and without the application of long wavelength radiationsources. The rewettable asymmetric membranes formed herein exhibit highflux and good salt rejections.

Porous substrates generally have a network of interconnecting passagesor pores extending from one surface to the other. These interconnectingpassages provide a tortuous passageway through which liquids can passduring the process of being filtered.

In the method of the present disclosure, porous substrates generallyhave a first major surface, pores (e.g., interstitial), and a secondmajor surface. Suitable substrates can be selected from a variety ofmaterials so long as the porous substrate is coatable (e.g., capable ofhaving a polymerizable composition applied to at least a portion of thethickness of the substrate) or can be adapted to be coatable, andcomprises openings or pores. “First major surface” of the poroussubstrate refers to the surface typically closest in proximity to theultraviolet radiation source. The “second major surface” refers to thesurface of the substrate opposite the first major surface and istypically located at a distance furthest from the ultraviolet radiationsource.

Suitable porous substrates include, for example, films, porousmembranes, woven webs, nonwoven webs, hollow fibers, and the like Theporous substrate can be formed from polymeric materials, ceramicmaterials, and the like, or combinations thereof. Some suitablepolymeric materials include, for example, polyolefins, poly(isoprenes),poly(butadienes), fluorinated polymers, polyvinyl chlorides, polyesters,polyamides, polyimides, polyethers, poly(ether sulfones),poly(sulfones), poly(ether)sulfones, polyphenylene oxides, polyphenylenesulfides, poly(vinyl acetates), copolymers of vinyl acetate, poly(phosphazenes), poly(vinyl esters), poly(vinyl ethers), poly(vinylalcohols), poly(carbonates) and the like, or combinations thereof.Suitable polyolefins include, for example, poly(ethylene),poly(propylene), poly(1-butene), copolymers of ethylene and propylene,alpha olefin copolymers (such as copolymers of 1-butene, 1-hexene,1-octene, and 1-decene), poly(ethylene-co-1-butene),poly(ethylene-co-1-butene-co-1-hexene), and the like, or combinationsthereof. Suitable fluorinated polymers include, for example, poly(vinylfluoride), poly(vinylidene fluoride), copolymers of vinylidene fluoride(such as poly(vinylidene fluoride-co-hexafluoropropylene)), copolymersof chlorotrifluoroethylene (such aspoly(ethylene-co-chlorotrifluoroethylene)), and the like, orcombinations thereof. Suitable polyamides include, for example,poly(imino(1-oxohexamethylene)), poly(iminoadipoylimino hexamethylene),poly(iminoadipoyliminodecamethylene), polycaprolactam, and the like, orcombinations thereof. Suitable polyimides include, for example,poly(pyromellitimide), and the like. Suitable poly(ether sulfone)sinclude, for example, poly(diphenylether sulfone),poly(diphenylsulfone-co-diphenylene oxide sulfone), and the like, orcombinations thereof.

In some embodiments, the porous substrate can have an average pore sizeless than about 10 micrometers. In other embodiments, the average poresize of the porous substrate can be less than about 5 micrometers, lessthan about 2 micrometers, or less than about 1 micrometer. In otherembodiments, the average pore size of the porous substrate can begreater than about 10 nanometers. In some embodiments, the average poresize of the porous substrate is greater than about 50 nanometers,greater than about 100 nanometers, or greater than about 200 nanometers.In some embodiments, the porous substrate can have an average pore sizein a range of about 10 nanometers to about 10 micrometers, in a range ofabout 50 nanometers to about 5 micrometers, in a range of about 100nanometers to about 2 micrometers, or in a range of about 200 nanometersto about 1 micrometer.

Some suitable porous substrates include, for example, nanoporousmembranes, microporous membranes, microporous nonwoven webs, microporouswoven webs, microporous fibers, and the like. In some embodiments, theporous substrate can have a combination of different pore sizes (e.g.,micropores, nanopores, and the like). In one embodiment, the poroussubstrate is microporous. In some embodiments, the porous substrate cancomprise a particulate or a plurality of particulates.

The thickness of the porous substrate selected can depend on theintended application of the membrane. Generally, the thickness of theporous substrate can be greater than about 10 micrometers. In someembodiments, the thickness of the porous substrate can be greater thanabout 1,000 micrometers, or greater than about 10,000 micrometers.

In some embodiments, the porous substrate is hydrophobic. In anotherembodiment, the porous substrate is hydrophilic. The porous substrateeither being hydrophobic or hydrophilic can be coated with apolymerizable composition and exposed to an ultraviolet radiation sourceas described below.

In some embodiments, the porous substrate comprises a microporous,thermally-induced phase separation (TIPS) membrane. TIPS membranes canbe prepared by forming a solution of a thermoplastic material and asecond material above the melting point of the thermoplastic material.Upon cooling, the thermoplastic material crystallizes and phaseseparates from the second material. The crystallized material can bestretched. The second material can be optionally removed either beforeor after stretching. TIPS membranes are disclosed in U.S. Pat. No.1,529,256 (Kelley); U.S. Pat. No. 4,726,989 (Mrozinski); U.S. Pat. No.4,867,881 (Kinzer); U.S. Pat. No. 5,120,594 (Mrozinski); U.S. Pat. No.5,260,360 (Mrozinski); U.S. Pat. No. 5,962,544 (Waller, Jr.); and U.S.Pat. No. 4,539,256 (Shipman). In some embodiments, TIPS membranescomprise polymeric materials such as poly(vinylidene fluoride) (i.e.,PVDF), polyolefins such as poly(ethylene) or poly(propylene),vinyl-containing polymers or copolymers such as ethylene-vinyl alcoholcopolymers and butadiene-containing polymers or copolymers, andacrylate-containing polymers or copolymers. TIPS membranes comprisingPVDF are further described in U.S. Patent Application Publication No.2005/0058821 (Smith et al.)

In some embodiments, the porous substrate can be a nonwoven web havingan average pore size that is typically greater than about 10micrometers. Suitable nonwoven webs include, for example, melt-blownmicrofiber nonwoven webs described in Wente, V. A., “SuperfineThermoplastic Fibers”; Industrial Engineering Chemistry, 48, 1342-1346(1956), and Wente, V. A., “Manufacture of Super Fine Organic Fibers”;Naval Research Laboratories (Report No. 4364) May 25, 1954. In someembodiments, suitable nonwoven webs can be prepared from nylon.

Some examples of suitable porous substrates include commerciallyavailable materials such as hydrophilic and hydrophobic microporousmembranes known under the trade designations DURAPORE and MILLIPOREEXPRESS MEMBRANE, available from Millipore Corporation of Billerica,Mass. Other suitable commercial microporous membranes known under thetrade designations NYLAFLO and SUPOR are available from Pall Corporationof East Hills, New York.

In the method of the present disclosure, a polymerizable species isapplied to the porous substrate. The term “polymerizable composition”generally refers to compositions having at least one polymerizablespecies, at least one copolymer comprising hydrophilic and hydrophobicgroups, a solvent, and at least one photoinitiator. The polymerizablespecies can be polymerized on the first major surface, within the poresor at least a portion of the pores, or on the second major surface ofthe porous substrate when exposed to an ultraviolet radiation source.The copolymer comprising hydrophilic and hydrophobic groups can coat theinterstitial pores of the porous substrate. The copolymer can residewith the polymerized material throughout a portion of the thickness ofthe substrate. The copolymer can collect on the remainder of theinterstitial pores where the concentration of the polymerized materialis negligible. The second major surface is substantially free of thepolymerized material. The solvent is selected to dissolve, suspend, ordisperse the polymerizable species, the copolymer, and thephotoinitiator or combinations thereof of the polymerizable composition.

The photoinitator can be selected for initiating the polymerization ofthe polymeric species and can selectively absorb radiation fromultraviolet radiation sources. In some embodiments, the polymerizablecomposition applied to the porous substrate doesn't require aphotoinitiator as described in U.S. Pat. No. 5,891,530 (Wright).

The polymerizable composition can be applied to at least a portion ofthe thickness of the porous substrate. The polymerizable composition,after exposure to the ultraviolet radiation source, can form polymerizedmaterial extending through at least a portion of the thickness of theporous substrate. The resulting polymerized material together with thecopolymer can reside on the first major surface and within the poroussubstrate by chemical or physical interactions. The second major surfacecan be coated with the copolymer and be substantially free of thepolymerized material. In some embodiments, the polymerized materialand/or copolymer can graft onto the surfaces of the porous substrate. Inanother embodiment, the polymerized material and/or copolymer residewithin and on the surfaces of the interstitial pores of the poroussubstrate through hydrogen bonding, Van der Waals interactions, ionicbonding, and the like.

The photoinitiator of the polymerizable composition can initiatepolymerization of the polymerizable species. The polymerizablecomposition can comprise about 0.001 to about 5.0 weight percentphotoinitiator. Some suitable photoinitiators can include, for example,organic compounds, organometallic compounds, inorganic compounds, andthe like. Some examples of free radical photoinitiators include, forexample, benzoin and its derivatives, benzyl ketals, acetophenone,acetophenone derivatives, benzophenone, and benzophenone derivatives,acyl phosphine oxides, and the like, or combinations thereof. In someembodiments, some photoinitiators (e.g., acyl phosphine oxides) canabsorb long wavelength ultraviolet radiation, short wavelengthultraviolet radiation, and the like or combinations thereof.

Exemplary photoinitiators for initiating free-radical polymerization of(meth)acrylates, for example, include benzoin and its derivatives suchas alpha-methylbenzoin; alpha-phenylbenzoin; alpha-allylbenzoin;alpha-benzylbenzoin; benzoin ethers such as benzil dimethyl ketal(available, for example, under the trade designation IRGACURE 651 fromCiba Specialty Chemicals, Tarrytown, N.Y.), benzoin methyl ether,benzoin ethyl ether, benzoin n-butyl ether; acetophenone and itsderivatives such as 2-hydroxy-2-methyl-1-phenyl-1-propanone (available,for example, under the trade designation DAROCUR 1173 from CibaSpecialty Chemicals) and 1-hydroxycyclohexyl phenyl ketone (available,for example, under the trade designation IRGACURE 184 from CibaSpecialty Chemicals);2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone(available, for example, under the trade designation IRGACURE 907 fromCiba Specialty Chemicals);2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone(available, for example, as IRGACURE 369 from Ciba Specialty Chemicals).Other useful photoinitiators include pivaloin ethyl ether, anisoin ethylether; anthraquinones, such as anthraquinone, 2-ethylanthraquinone,1-chloroanthraquinone, 1,4-dimethylanthraquinone,1-methoxyanthraquinone, benzanthraquinonehalomethyltriazines;benzophenone and its derivatives; iodonium salts and sulfonium salts asdescribed hereinabove; titanium complexes such asbis(eta₅-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium(obtained under the trade designation CGI 784 DC, also from CibaSpecialty Chemicals); halomethylnitrobenzenes such as, for example,4-bromomethylnitrobenzene; mono- and bis-acylphosphines (available, forexample, from Ciba Specialty Chemicals as IRGACURE 1700, IRGACURE 1800,IRGACURE 1850, and DAROCUR 4265).

The photoinitiator of the polymerizable composition is selected toinitiate polymerization of polymerizable species throughout at least aportion of the thickness of the porous substrate. The thickness of theporous substrate extends from the first major surface to the secondmajor surface. The photoinitiator can initiate polymerization of thepolymerizable species upon exposure to the ultraviolet radiation sourceat the first major surface, and can extend through a portion of thethickness of the porous substrate. The initiation of polymerizablespecies for forming polymerized material can decrease through thethickness to the second major surface.

Polymerizable species (e.g., monomers) of the polymerizable compositioncan polymerize by many polymerization routes. In particular, thepolymerizable species can attach to reactive groups or polymerizablespecies on the substrate by chemical bonding (e.g., free radicalreaction) to form a covalent bond. The resulting surface properties ofthe rewettable asymmetric membrane are typically different than thesurface properties of the initial porous substrate. For example, theaddition of polymerized material and a copolymer to the porous substrateprovides for reactive surfaces when contacted by other species, forexample, by interactions including hydrogen bonding, Van der Waalsinteractions, ionic bonding, and the like.

In some embodiments, the polymerizable species of the polymerizablecomposition is a monomer having a free-radically polymerizable group. Insome embodiments, the polymerizable species may comprise afree-radically polymerizable group and an additional functional groupthereon. The free-radically polymerizable group can be an ethylenicallyunsaturated group such as a (meth)acryloyl group, an acryoyl group, or avinyl group. The free-radically polymerizable group, after initiation bya photoinitiator, can polymerize within the porous substrate forming apolymerized material upon exposure to the ultraviolet radiation source.The reaction of the free-radically polymerizable groups of thepolymerizable species with other reactive groups or polymerizablespecies of the porous substrate upon exposure to ultraviolet radiationcan result in the formation of a greater concentration of thepolymerized material at the first major surface and within the openingsor pores nearest the first major surface than at the second majorsurface of the rewettable asymmetric membrane.

In addition to having a free-radically polymerizable group,polymerizable species can contain a second or additional functionalgroup. In some embodiments, the second functional group is selected froma second ethylenically unsaturated group, ring opening groups (e.g.,epoxy group, an azlactone group, and an aziridine group), an isocyanatogroup, an ionic group, an alkylene oxide group, or combinations thereof.The second or additional functional group of the polymerizable speciescan provide for further reactivity or affinity of the polymerizedmaterial retained within the porous substrate. In some embodiments, theadditional functional group can react to form a linking group betweenthe porous substrate and other material such as other species ornucleophilic compounds having at least one nucleophilic group.

The presence of an additional functional group can impart a desiredsurface property to the rewettable asymmetric membrane such as anaffinity for a particular type of compound. In some embodiments, thepolymerizable species can contains an ionic group such that therewettable asymmetric membrane containing polymerized material and acopolymer can often have an affinity for compounds having an oppositecharge. That is, compounds with negatively charged groups can beattracted to the rewettable asymmetric membrane having polymerizedmaterial with a cationic group and compounds with positively chargedgroups can be attracted to a the rewettable asymmetric membrane havingpolymerized material with an anionic group. Further, the polymerizedmaterial can modify the hydrophobic character of the initial substrateand impart a hydrophilic property to at least one major surface of therewettable asymmetric membrane. In one embodiment, the polymerizedmaterial containing an alkylene oxide group can impart hydrophiliccharacter to the rewettable asymmetric membrane.

In still other embodiments, suitable polymerizable species of thepolymerizable composition can have a free-radically polymerizable groupthat is an ethylenically unsaturated group and an additional functionalgroup that is an ionic group. The ionic group can have a positivecharge, a negative charge, or a combination thereof. With some suitableionic species, the ionic group can be neutral or charged depending onthe pH conditions. This class of species is typically used to impart adesired surface affinity for one or more oppositely charged compounds orto decrease the affinity for one or more similarly charged compounds.

In still other embodiments, suitable ionic polymerizable species havinga negative charge include (meth)acrylamidosulfonic acids of Formula I orsalts thereof

In Formula I, R¹ is hydrogen or methyl; and Y is a straight or branchedalkylene (e.g., alkylenes having 1 to 10 carbon atoms, 1 to 6 carbonatoms, or 1 to 4 carbon atoms). Exemplary ionic species according toFormula I include, but are not limited to, N-acrylamidomethanesulfonicacid, 2-acrylamidoethanesulfonic acid,2-acrylamido-2-methyl-1-propanesulfonic acid, and2-methacrylamido-2-methyl-1-propanesulfonic acid. Salts of these acidicspecies can also be used. Counter ions for the salts can be, forexample, ammonium ions, potassium ions, lithium ions, or sodium ions.

Other suitable ionic polymerizable species having a negative chargeinclude sulfonic acids such as vinylsulfonic acid and 4-styrenesulfonicacid; (meth)acrylamidophosphonic acids such as(meth)acrylamidoalkylphosphonic acids (e.g., 2-acrylamidoethylphosphonicacid and 3-methacrylamidopropylphosphonic acid); acrylic acid andmethacrylic acid; and carboxyalkyl(meth)acrylates such as2-carboxyethylacrylate, 2-carboxyethylmethacrylate,3-carboxypropylacrylate, and 3-carboxypropylmethacrylate. Still othersuitable acidic species include (meth)acryloylamino as described in U.S.Pat. No. 4,157,418 (Heilmann et al). Exemplary (meth)acryloylamino acidsinclude, but are not limited to, N-acryloylglycine, N-acryloylasparticacid, N-acryloyl-β-alanine, and 2-acrylamidoglycolic acid. Salts of anyof these acidic species can also be used.

Other ionic polymerizable species that are capable of providing apositive charge are amino (meth)acrylates or amino (meth)acrylamides ofFormula II or quaternary ammonium salts thereof. The counter ions of thequaternary ammonium salts are often halides, sulfates, phosphates,nitrates, and the like.

In Formula II, R¹ is hydrogen or methyl; L is oxy or —NH—; and Y is analkylene (e.g., an alkylene having 1 to 10 carbon atoms, 1 to 6, or 1 to4 carbon atoms). Each R² is independently hydrogen, alkyl, hydroxyalkyl(i.e., an alkyl substituted with a hydroxy), or aminoalkyl (i.e., analkyl substituted with an amino). Alternatively, the two R² groups takentogether with the nitrogen atom to which they are attached can form aheterocyclic group that is aromatic, partially unsaturated (i.e.,unsaturated but not aromatic), or saturated, wherein the heterocyclicgroup can optionally be fused to a second ring that is aromatic (e.g.,benzene), partially unsaturated (e.g., cyclohexene), or saturated (e.g.,cyclohexane).

In some embodiments of Formula II, both R² groups are hydrogen. In otherembodiments, one R² group is hydrogen and the other is an alkyl having 1to 10, 1 to 6, or 1 to 4 carbon atoms. In still other embodiments, atleast one of R² groups is a hydroxy alkyl or an amino alkyl that have 1to 10, 1 to 6, or 1 to 4 carbon atoms with the hydroxy or amino groupbeing positioned on any of the carbon atoms of the alkyl group. In yetother embodiments, the R² groups combine with the nitrogen atom to whichthey are attached to form a heterocyclic group. The heterocyclic groupincludes at least one nitrogen atom and can contain other heteroatomssuch as oxygen or sulfur. Exemplary heterocyclic groups include, but arenot limited to imidazolyl. The heterocyclic group can be fused to anadditional ring such as a benzene, cyclohexene, or cyclohexane.Exemplary heterocyclic groups fused to an additional ring include, butare not limited to, benzoimidazolyl.

Exemplary amino (meth)acrylates (i.e., L in Formula II is oxy) include,for example, N,N-dialkylaminoalkyl(meth)acrylates such as, for example,N,N-dimethylaminoethylmethacrylate, N,N-dimethylaminoethylacrylate,N,N-diethylaminoethylmethacylate, N,N-diethylaminoethylacrylate,N,N-dimethylaminopropylmethacrylate, N,N-dimethylaminopropylacrylate,N-tert-butylaminopropylmethacrylate, N-tert-butylaminopropylacrylate andthe like.

Exemplary amino (meth)acrylamides (i.e., L in Formula II is —NH—)include, for example, N-(3-aminopropyl)methacrylamide,N-(3-aminopropyl)acrylamide, N-[3-(dimethylamino)propyl]methacrylamide,N-(3-imidazolylpropyl)methacrylamide, N-(3-imidazolylpropyl)acrylamide,N-(2-imidazolylethyl)methacrylamide,N-(1,1-dimethyl-3-imidazoylpropyl)methacrylamide,N-(1,1-dimethyl-3-imidazoylpropyl)acrylamide,N-(3-benzoimidazolylpropyl)acrylamide, andN-(3-benzoimidazolylpropyl)methacrylamide.

Exemplary quaternary salts of the ionic species of Formula II include,but are not limited to, (meth)acrylamidoalkyltrimethylammonium salts(e.g., 3-methacrylamidopropyltrimethylammonium chloride and3-acrylamidopropyltrimethylammonium chloride) and(meth)acryloxyalkyltrimethylammonium salts (e.g.,2-acryloxyethyltrimethylammonium chloride,2-methacryloxyethyltrimethylammonium chloride,3-methacryloxy-2-hydroxypropyltrimethylammonium chloride,3-acryloxy-2-hydroxypropyltrimethylammonium chloride, and2-acryloxyethyltrimethylammonium methyl sulfate).

Other polymerizable species can be selected from those known to providepositively charged groups, for example, to an ion exchange resin. Suchpolymerizable species include, for example, the dialkylaminoalkylamineadducts of alkenylazlactones (e.g., 2-(diethylamino)ethylamine,(2-aminoethyl)trimethylammonium chloride, and3-(dimethylamino)propylamine adducts of vinyldimethylazlactone) anddiallylamine species (e.g., diallylammonium chloride anddiallyldimethylammonium chloride).

In some methods for making a rewettable asymmetric membrane, suitablepolymerizable species can have two free-radically polymerizable groupsas well as a hydrophilic group. For example, alkylene glycoldi(meth)acrylates can be used as polymerizable species to imparthydrophilic character to a hydrophobic porous substrate. Thesepolymerizable species have two (meth)acryloyl groups and a hydrophilicpolyalkylene glycol (i.e., polyalkylene oxide) group.

When the membrane has polymerizable species that contains an epoxygroup, an azlactone group, or an isocyanato group, the rewettableasymmetric membrane can be further treated such that the functionalgroups can react with a nucleophilic compound having a one or aplurality of nucleophilic groups to impart a hydrophilic character to ahydrophobic porous substrate. Unreacted nucleophilic groups cancontribute to forming a hydrophilic functionalized membrane. Someexemplary nucleophilic compounds contain a hydrophilic group such as apolyalkylene oxide group in addition to the nucleophilic group. Forexample, the nucleophilic compound such as polyalkylene glycol diaminesand polyalkylene glycol triamines can include a plurality of aminogroups.

Polymerizable compositions of the present disclosure can be prepared,for example, as a coatable solution, dispersion, emulsion, or the like.The polymerizable compositions can be applied to the first majorsurface, interstitial pores, and the second major surface of the poroussubstrate. In some examples, the porous substrate can be saturated orimmersed with a polymerizable composition comprising at least onepolymerizable species, at least one copolymer comprising hydrophilicand/or hydrophobic groups, at least one solvent and at least onephotoinitiator. The concentration of the polymerizable species, forexample, can vary depending on a number of factors including, but notlimited to, the polymerizable species, the copolymer, the extent ofpolymerization or crosslinking of the polymerizable species on andwithin the porous substrate, the reactivity of the polymerizablespecies, the crosslinker concentration, or the solvent used. In someembodiments, the concentration of the polymerizable species of thepolymerizable composition can be in a range of about 2 weight percent toabout 99.9 weight percent.

In one embodiment, the porous substrate can have a hydrophilic surfaceprior to contacting the polymerizable composition. After contacting thepolymerizable composition with an ultraviolet radiation source, thehydrophobic surface can impart a hydrophobic property to at least onesurface of the rewettable asymmetric membrane.

In some embodiments, the polymerizable species of the polymerizablecomposition have a free-radically polymerizable group that is a firstethylenically unsaturated group and a second functional group that is asecond ethylenically unsaturated group. In one embodiment, thepolymerizable composition includes a crosslinker suitable forcrosslinking the polymerizable species forming a network or gelledpolymerized material. Suitable polymerizable species having twoethylenically unsaturated groups include, but are not limited to,polyalkylene glycol di(meth)acrylates. The term polyalkylene glycoldi(meth)acrylate is used interchangeably with the term polyalkyleneoxide di(meth)acrylate. The term “(meth)acryl” as in (meth)acrylate isused to encompass both acryl groups as in acrylates and methacryl groupsas in methacrylates. Exemplary polyalkylene glycol di(meth)acrylatesinclude polyethylene glycol di(meth)acrylate species and polypropyleneglycol di(meth)acrylate species. Polyethylene glycol diacrylate specieshaving an average molecular weight of about 400 g/mole is commerciallyavailable, for example, under the trade designation SR344 andpolyethylene glycol dimethacrylate species having an average molecularweight of about 400 g/mole is commercially available under the tradedesignation SR603 from Sartomer Company, Incorporated of Exton,Pennsylvania.

In some embodiments, suitable polymerizable species have afree-radically polymerizable group that is a first ethylenicallyunsaturated group and an additional functional group that is an epoxygroup. Suitable polymerizable species within this class include, but arenot limited to, glycidyl (meth)acrylates. This class of polymerizablespecies can provide a functionalized rewettable asymmetric membranehaving at least one epoxy group available for further reactivity. Theepoxy group can react with other reactants such as with another speciesor with a nucleophilic compound to impart a desired surface property tothe porous substrate (e.g., affinity for a particular compound orfunctional group having different reactivity). The reaction of the epoxygroup with a nucleophilic compound, for example, results in the openingof the epoxy ring and the formation of a linkage group that functions totether the nucleophilic compound to the porous substrate. Suitablenucleophilic groups for reacting with epoxy groups include, but are notlimited to, primary amino groups, secondary amino groups, and carboxygroups. The nucleophilic compound can contain more than one nucleophilicgroup that can crosslink multiple epoxy groups or more than one optionalgroups that can impart hydrophilic character to the functionalizedmembrane. The linkage group formed by ring-opening of the epoxy groupoften contains the group —C(OH)HCH₂NH— when the epoxy is reacted with aprimary amino group or —C(OH)HCH₂O(CO)— when the epoxy is reacted with acarboxy group.

In some instances, the epoxy groups of the polymerized material withinthe porous substrate can be reacted with a multifunctional amine such asa diamine having two primary amino groups or a triamine having threeprimary amino groups. One of the amino groups can undergo a ring openingreaction with the epoxy group and result in the formation of a linkagegroup that contains the group —C(OH)HCH₂NH— between the nucleophiliccompound and the porous substrate. The second amino group or the secondand third amino groups can impart a hydrophilic character to therewettable asymmetric membrane or can crosslink two or morepolymerizable species by reacting with one or more additional epoxygroups. In some examples, the multifunctional amine is a polyalkyleneglycol diamine or polyalkylene glycol triamine and reaction with anepoxy group results in the attachment of a polymerized material having apolyalkylene glycol group (i.e., polyalkylene oxide group). Thepolyalkylene glycol group as well as any terminal primary amino grouptends to impart hydrophilic character to the rewettable asymmetricmembrane.

In still other embodiments, suitable polymerizable species have afree-radically polymerizable group that is an ethylenically unsaturatedgroup and an additional functional group that is an azlactone group.Suitable polymerizable species include, but are not limited to, vinylazlactone such as 2-vinyl-4,4-dimethylazlactone. This class ofpolymerizable species can provide a rewettable asymmetric membranehaving at least one azlactone group available for further reactivity.The azlactone group can react with other reactants such as anotherspecies or with a nucleophilic compound to impart a desired surfaceproperty to the porous substrate (e.g., affinity for a particularcompound or functional group having different reactivity). The reactionof the azlactone group with a nucleophilic compound, for example,results in the opening of the azlactone ring and the formation of alinkage group that functions to attach the nucleophilic compound to theporous substrate. The nucleophilic compound typically contains at leastone nucleophilic group. Suitable nucleophilic groups for reacting withan azlactone group include, but are not limited to, primary aminogroups, secondary amino groups and hydroxy groups. The nucleophiliccompound can contain additional nucleophilic groups that can crosslinkmultiple azlactone groups or can contain other optional groups that canimpart a hydrophilic character to the rewettable asymmetric membrane.The linkage group formed by ring-opening of the azlactone group oftencontains the group —(CO)NHCR₂(CO)— where R is an alkyl such as methyland (CO) denotes a carbonyl.

In some instances, the azlactone groups can be reacted with amultifunctional amine such as a diamine having two primary amino groupsor a triamine having three primary amino groups. One of the amino groupscan undergo a ring opening reaction with the azlactone group and resultin the formation of a linkage containing the group —(CO)NHCR₂(CO)—between the nucleophilic compound and the porous substrate. The secondamino group or second and third amino groups can impart a hydrophiliccharacter to the rewettable asymmetric membrane or can crosslinkmultiple polymerizable species. In some examples, the multifunctionalamine is a polyalkylene glycol diamine or a polyalkylene glycol triamineand reaction with an azlactone group results in the attachment of apolymerizable species having a polyalkylene glycol group (i.e.,polyalkylene oxide group). The polyalkylene glycol group as well as anyterminal primary amino group tends to impart a hydrophilic character tothe rewettable asymmetric membrane.

In still other embodiments, suitable polymerizable species can have afree-radically polymerizable group that is an ethylenically unsaturatedgroup and an additional functional group that is an isocyanato group.Some suitable polymerizable species include, but are not limited to anisocyanatoalkyl (meth)acrylate such as 2-isocyanatoethyl methacrylateand 2-isocyanatoethyl acrylate. This class of polymerizable species canprovide the rewettable asymmetric membrane having at least one reactiveisocyanato group. The isocyanato group can react with other reactantssuch as another species or with a nucleophilic compound to impart adesired surface property to the rewettable asymmetric membrane (e.g.,affinity for a particular compound or functional group having differentreactivity). The reaction of an isocyanato group with a nucleophiliccompound can result in the formation of a urea linkage if thenucleophilic group is a primary amino or secondary amino group or in theformation of a urethane linkage if the nucleophilic group is a hydroxygroup. The nucleophilic compound can contain additional nucleophilicgroups that can crosslink multiple isocyanato groups or can containother optional groups that can impart a hydrophilic character to therewettable asymmetric membrane. The linkage group formed by reaction ofa nucleophilic compound with an isocyanato group often contains thegroup —NH(CO)NH— when the nucleophilic group is a primary amino group or—NH(CO)O— when the nucleophilic group is a hydroxy.

Copolymers of the polymerizable composition can coat the interstitialpores, the first major surface and the second major surface of theporous substrate. The copolymer is combined with the polymerizedmaterial to coat the interstitial pores and to collect on the remainderof the interstitial pores of the porous substrate of the rewettableasymmetric membrane. In some embodiments, a mixture of the copolymerwith the polymerized material resides within the interstitial pores. Insome embodiments, the copolymer can coat or collect on the interstitialpores such that the polymerized material has the copolymer coatedbetween the polymerized material and the porous substrate.

Copolymers useful for forming the rewettable asymmetric membrane havehydrophilic and hydrophobic groups which include copolymers which aregenerally water-swellable, water insoluble, and can provide for adurable coating on the porous substrate. Some examples of copolymershaving these groups can include, for example, cellulose derivatives suchas cellulose acetate, cellulose acetate butyrate, cellulose acetatepropionate, 2-hydroxyethyl cellulose, and ethyl cellulose; polyesterssuch as poly(ethylene adipate), polyethylene glycol terephthalate,poly(L-lactide), poly(DL-lactide) and poly(DL-Lactide-co-glycolide);polyamides such as poly(hexamethyleneadipamide) andpoly(hexamethylenesebacamide; polyacrylates such as poly(2-hydroxyethylmethacrylate) and poly(2-hydroxyporpyl methacrylate); ethylene-vinylalcohol copolymers; poly(ethylene-co-allyl alcohol); polyhydroxystyrene;and poly(vinyl alcohol). Further examples of copolymers of thepolymerizable composition can include water insoluble charged copolymersincluding, for example, sulfonated poly(ether-ether-ketone) ((S-PEEK),having a sulfonation less than 86%); sulfonated poly(phenylene oxide)((S—PPO), having a sulfonation less than 70%), sulfonatedpoly(ether)sulfone, and sulfonated polystyrene. Examples further includeaminated polysulfone, aminated poly(phenylene oxide), aminatedpoly(vinylbenzyl chloride), partially protonated or alkylatedpoly(4-vinylpyridine). Neutral and ionically charged random and blockcopolymers are other examples of suitable copolymers.

Some hydrophobic monomers can be polymerized with other hydrophobicmonomers for forming copolymers useful in the present disclosure. Insome embodiments, more than one hydrophobic monomer can be polymerizedwith more than one hydrophilic monomer, and the like. Examples ofhydrophobic monomers can include, for example, methyl methacrylate(MMA); iso-, sec-, tert- or n-propyl (meth)acrylate; iso-, sec-, tert-or n-butyl (meth)acrylate; n-hexyl acrylate; n-heptyl methacrylate;1-hexadecyl methacrylate; n-myristyl acrylate; n-octyl methacrylate;stearyl acrylate; 3,3,5-trimethylcyclohexyl methacrylate; and undecyl(meth)acrylate. Further hydrophobic monomers can include vinyl laurate;vinyl stearate; tert-amyl methacrylate; cyclohexyl(meth)acrylate; n- oriso-decyl meth(acrylate); di(n-butyl)itaconate; n-dodecyl methacrylate;2-ethylbutyl methacrylate; 2-ethylhexyl acrylate; isooctyl acrylate;isotridecylacrylate; isobornyl acrylate; vinyl butyrate; N-ethylmethacrylamide; N-tert-butylacrylamide; N-(n-octadecyl)acrylamide;N-tert-octylacrylamide N-benzylmethacrylamide; N-cyclohexylacrylamide;N-diphenylmethylacrylamide; N-dodecylmethacrylamide; styrene; 2-, 3-, or4-methylstyrene; vinyl octadecylether; and vinyl iso-octyl ether, andthe like. Some examples of hydrophilic monomers can include4-hydroxybutyl methacrylate; 2-hydroxyethyl (meth)acrylate;hydroxypropyl (meth)acrylate; poly(ethylene glycol) mono (meth)acylates;poly(propylene glycol)mono (meth)acrylates; glycerol mono(meth)acrylate;2-(2-ethoxyethoxy) ethyl acrylate; tetrahydrofurfuryl acrylate;N-acryloyltri(hydroxymethyl)methylamine; monoacrylkoxyethyl phosphate;1,1,1-trimethylolporpane diallyl ether; 1,1,1-trimethlolpropane diallylether; 1,1,1-trimethylolpropane monoallyl ether;vinyl-4-hydroxybutylether; (meth)acrylamide; n-isopropyl acrylamide;N-vinylformamide; N-vinyl-N-methacetamide;N-(2-hydroxypropyl)methacrylamide; N,N-diethyl(meth)acrylamide;N,N-dimethyl (meth)acrylamide; N-methylmethacrylamide;N-methlolacrylamide; N-vinyl-2-pyrrolidone; N-vinylcaprolactam; vinylmethylsulfone; N-vinylurea; and N-(meth)acryloylmorpholine.

Some examples of charged monomers useful for forming copolymers include,for example, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and saltforms; sodium sulfonate; vinylsulfonic acid; acrylamidoglycolic acid;(meth)acrylic acid; itaconic acid; 2-propene-s-sulfonic acid sodiumacrylate; 2-sulfonethyl (meth)acrylate; 3-sulfopropyl (meth)acrylate;vinylbenzioic acid; vinylsulfonic acid; 2-carboxyethyl acrylate;(Meth)acrylamidopropyltrimethylammonium chloride (MAPTAC/APTAC);2-methacryloxyethyltrimethylammonium chloride; methacryloylchloinemethyl sulphate; 2-N-morpholinoethyl acrylate; 2-N-morpholinoethylmethacrylate; 1-vinylimidazole, 2- or 4-vinylpyridine;2-acryloxyethyltrimethylammonium chloride; 2-aminoethyl methacrylatehydrochloride; N-(tert-butylamino)ethyl methacrylate; diallylamine;diallyldimethylammonium chloride; 2-(N,N-diethylamino) ethylmethacrylate; 2-(diethylamino)ethylstyrene; 2-(N,N-dimethylamino)ethylacrylate; N-[2-(N,N-dimethylamino)ethyl]methacrylamide;2-(N,N-dimethylamino)ethyl and methacrylate;N-[2-(N,N-dimethylamino)propyl)(meth)acrylamide.

In some embodiments, copolymers can contain functional groups such thatthe crosslinking or other chemical reactions can occur. Some examples ofreactive monomers for copolymers can include, for example, methacrylicanhydride, vinyl azlactone, acrylic anhydride, allyl glycidyl ether,allylsuccinic anhydride, 2-cinnamoyloxyethyl (meth)acrylate, cinnamyl(meth)acrylate, citraconic anhydride, and glycidyl acrylate,

Some examples of random copolymers having hydrophilic and hydrophobicmonomers include, poly(AMPS-co-N-t-butylacrylamide),poly(APTAC-co-N-t-butylacrylamide),poly(N-vinylformamide-co-N-t-butylacrylamide), and poly(AMPS-co-MMA),and the like.

Copolymers having hydrophilic and hydrophobic groups are commerciallyavailable. The groups of the copolymer can be chemically or physicallymodified to form the hydrophilic or hydrophobic block of the copolymerprior to or after application to the porous substrate. The copolymer canbe dissolved, dispersed, or suspended in the polymerizable composition.

In some embodiments, the hydrophobic block can be a polyolefin and thehydrophilic block can be a polyacetate. In a preferred embodiment, thecopolymer can be an ethylene-vinyl alcohol copolymer. The hydrophilicblock of the ethylene-vinyl alcohol copolymer can be chemicallymodified.

Ethylene-vinyl alcohol copolymers are generally formed fromethylene-vinyl acetate copolymers after saponification. Theethylene-vinyl acetate copolymer comprises ethylene and vinyl acetatemonomers. After saponification of the ethylene-vinyl acetate copolymer,the vinyl acetate units can be chemically modified to vinyl alcoholunits. Other monomer components can also be present in the saponifiedethylene-vinyl acetate copolymer in such amounts not to impair thehydrophilicity of the hydrophilic membrane. The ethylene-vinyl alcoholcopolymer can be of various types including, for example, randomcopolymers, block copolymers, graft copolymers, and the like, orcombinations thereof. Similarly, the selection of ethylene-vinyl alcoholcopolymer can depend on the structure and molecular weight of theethylene-vinyl acetate copolymer formed prior to saponification.

In some embodiments, the polymerizable composition comprising acopolymer further comprises a solvent. The solvent can comprise a liquidsuitable for dissolving, dispersing or suspending the polymerizablespecies and/or copolymer of the polymerizable composition.

In one embodiment, the copolymer can be dissolved in the polymerizablespecies.

In another embodiment, the copolymer is an ethylene-vinyl alcoholcopolymer. The solvent for the ethylene-vinyl alcohol copolymer can be acombination of water and an organic liquid. The organic liquid selectedcan be miscible with water.

In one embodiment further comprising a solvent, some or substantiallymost of the solvent can be removed from the porous substrate. As thesolvent is removed from the porous membrane, the copolymer can coat atleast a portion of the surface and the pores of the porous substrate. Insome embodiments, at least 50 weight percent of the organic solvent canbe removed. In another embodiment, at least 60 weight percent, at least70 weight percent, at least 80 weight percent, or at least 90 weightpercent of the organic solvent can be removed from the porous substrate.After removing the solvent, for example, by evaporation, the rewettableasymmetric membrane can be washed with a solvent, and further dried.

In another embodiment, the rewettable asymmetric membrane after exposureto ultraviolet radiation can be exposed to a nonsolvent so that thecopolymer can collect on the interstitial pores of the membrane. In oneembodiment, the copolymer is the ethylene-vinyl alcohol copolymer. Therewettable asymmetric membrane after irradiation by the UV radiationsource can be immersed in a nonsolvent such as water (e.g., water bath).The water as a nonsolvent can cause a portion of the ethylene-vinylalcohol copolymer of the porous substrate to collect on the interstitialpores of the porous substrate. After exposure to the nonsolvent andcollection of the ethylene-vinyl alcohol copolymer on the poroussubstrate, the rewettable asymmetric membrane can be dried.

In some embodiments, the polymerizable composition can further include acrosslinker. Suitable crosslinkers can include difunctional andpolyfunctional acrylate and methacrylate free radically polymerizablemonomers. Some examples of crosslinkers can include, for example, esterderivatives of alkyl diols, triols, and tetrols (e.g., 1,4-butanedioldiacrylate, 1,6-hexanediol diacrylate, trimethylolpropane triacrylate,and pentaerythritol triacrylate). Some other difunctional andpolyfunctional acrylate and methacrylate monomers have been described inU.S. Pat. No. 4,379,201 (Heilmann et al.). In some embodiments,difunctional and polyfunctional acrylate monomers include, for example1,2-ethanediol diacrylate, 1,12-dodecanediol diacrylate, pentaerythritoltetracrylate, and the like, or combinations thereof. Difunctional andpolyfunctional acrylates and methacrylates can include acrylated epoxyoligomers, acrylated aliphatic urethane oligomers, acrylated polyetheroligomers, and acrylated polyester oligomers such as those commerciallyavailable under the trade designation EBECRYL from CYTEC SURFACESPECIALTIES of Smyrna, Georgia. Examples of other commercially availablemonomers as described above are available from Sartomer of Exton,Pennsylvania.

The polymerizable composition is applied to the porous substrate so asto coat, soak, wet, or immerse the porous substrate to provide a coatedporous substrate. The polymerizable composition can be applied to theporous substrate having a thickness extending from a first major surfaceto a second major surface of the porous substrate. The polymerizablecomposition can be applied to the porous substrate to wet or penetrateinto at least one micrometer of the thickness extending from the firstmajor surface. In some embodiments, the polymerizable composition canwet or penetrate through the entire thickness of the porous substrate.In some embodiments, the porous substrate can be immersed with thepolymerizable composition.

In some embodiments, the polymerizable composition can wet the surfacesof the pores throughout the thickness of the porous substrate to includewetting the first and second major surfaces. Suitable methods forapplying the polymerizable composition to the porous substrate include,for example, saturation or immersion techniques, spray coating, curtaincoating, slide coating, flood coating, die coating, roll coating,deposition, or by other known coating or application methods. Thepolymerizable composition to be applied to the porous substrategenerally has a viscosity such that the first major surface, the secondmajor surface and the pores of the porous substrate can be coated. Theviscosity of the polymerizable composition can be altered dependent onthe application method and the porous substrate chosen to receive thepolymerizable composition.

The coated porous substrate is exposed to an ultraviolet radiationsource to initiate and polymerize and/or crosslink the polymerizablecomposition. The ultraviolet radiation source selected for formingrewettable asymmetric membranes can depend on the intended processingconditions, and the appropriate energy source required for activatingthe photoinitiator present in the polymerizable composition forproviding a gradient concentration of polymerized material through thethickness of the porous substrate. Other considerations for selectingthe ultraviolet radiation source can include the amount and type ofpolymerizable species, crosslinker, and related materials (e.g.,copolymer) present in the polymerizable composition, the desired speedof the coated porous substrate as it moves past the ultraviolet sourcein a continuous manufacturing process, the distance of the poroussubstrate from the ultraviolet radiation source, and the thickness ofthe porous substrate.

A variety of ultraviolet (UV) radiation sources can be used to preparethe rewettable asymmetric membranes of the present disclosure. Suitablesources include low, medium- and high-pressure mercury arc lamps,electrodeless mercury lamps, light emitting diodes, mercury-xenon lamps,lasers and the like. Available ultraviolet radiation sources can bebroadband, narrowband or monochromatic. When broadband ultravioletradiation sources are used, filters can be applied to narrow thespectral output to a specific spectral region, thus eliminating certainwavelengths that can be detrimental to the rewettable asymmetricmembrane forming process. Suitable ultraviolet radiation sources are notrestricted by power and can be pulsed or continuous sources. Some ofthese radiation sources may or may not contain mercury. Preferredultraviolet radiation sources can be those that have relatively low IR(infrared) emissions that generally require no special coolingrequirements. Dichroic reflectors (cold mirrors) and/or dichroic frontwindows (hot mirrors), and/or water jackets and other methods know tothose skilled in the art can be used to help control the IR emissionsfrom the ultraviolet radiation source.

In one embodiment, the UV radiation source has a peak emissionwavelength less than 340 nm. In another embodiment, the UV radiationsource is a narrow bandwidth source.

In some embodiments, narrow bandwidth UV sources can be selected forwhich the UV radiation output spans a range of no more than about 50-100nm. One example of a narrow bandwidth UV radiation source includes, forexample, fluorescent ultraviolet lamps, which can operate withoutspecial filters and have low IR emissions. In a preferred embodiment,monochromatic or substantially monochromatic UV radiation sources suchas excimer lamps, lasers, light emitting diodes, and germicidal lampsare used. These sources have greater than 95% of their spectral outputconfined to a region spanning no more than about 20-30 nm. Some examplesof excimer lamps include a XeCl excimer lamp having a peak emission at308 nm, a KrCl excimer lamp having a peak emission at 222 nm, a Xe₂excimer lamp having a peak emission at 172 nm and a germicidal lamphaving a peak emission at 254 nm. Substantially monochromatic lampsproviding UV radiation output within a narrow spectral range and havinglow IR output are generally preferred. These lamps can allow for morecontrol in forming a gradient of polymerized material within thecopolymer retained within a rewettable asymmetric membrane and arecommercially available. Such sources are well known in the art. Anultraviolet radiation source can be a single source or a plurality ofsources. Similarly, the plurality of ultraviolet radiation sources canbe of the same source or of a combination of different ultravioletradiation sources.

Low and high power ultraviolet radiation sources (e.g., lamps) can beuseful for forming rewettable asymmetric membranes. Lamp power can beexpressed in watts/inch (W/in) based on the length of the lamp. Forexample, a high power lamp such as a 600 W/in electrodeless “H” bulb(Fusion UV Systems, Inc., Gaithersburg, Md.) is a 10-inch longmedium-pressure mercury bulb that can be excited by microwave energy. Atfull power, the 10 inch lamp requires a power supply rated at 6000 W todeliver power of 600 W/in. Such high power lamps can generate copiousamounts of UV radiation, but operate at lamp surface temperaturesexceeding 700° C. such that the ultraviolet output is accompanied bysignificant IR emissions. In contrast, a low power fluorescent UV lampcan operate at a typical power of 1-2 W/in, and requires less power tooperate having a surface temperature of about 43° C. to 49° C.

When exposing the coated porous substrate, the peak irradiance isgreater than 0 mW/cm² in the spectral region of the peak ultravioletintensity, and the spectral output must overlap with at least a portionof the absorption spectrum of the photoinitiator.

The UV spectrum is split into four primary spectral regions known asUVA, UVB, UVC and VUV, which are commonly defined as 315-400 nm, 280-315nm, 200-280 nm and 100-200 nm, respectively. The wavelength ranges citedherein are somewhat arbitrarily established, and may not correspond tothe exact wavelength ranges published by radiometer manufacturers fordefining the four primary spectral regions. Furthermore, some radiometermanufacturers specify that a UVV range (395-445 nm) that spans thetransition from UV to visible radiation.

In some instances, high power UV radiation sources can be employed Thesesources can have a peak irradiance of more than about 1 W/cm²accompanied by significant IR emissions. More preferred ultravioletradiation sources can comprise an array of germicidal or fluorescentbulbs providing a peak UV irradiance in the range from about 1-2 μW/cm²to 10-20 mW/cm². The peak irradiance from an array or a plurality ofmicrowave-driven fluorescent lamps commercially available from QuantumTechnologies of Irvine, Calif., can be as high as 50 mW/cm². The actualirradiance from an array of lamps can depend on a number of factorswhich include the electrical voltage, the lamp's power rating, the lampspacing within an array or plurality of lamps, the reflector(s) type (ifpresent), the age of the individual lamps, the transmission spectrum ofany windows or films through which the UV radiation must pass, thespecific radiometer used and its spectral responsivity, and the distanceof the array of lamps from the membrane.

The coated porous substrate can be exposed to the ultraviolet radiationsource for a period of time (e.g., exposure time) for polymerizing thepolymerizable composition to form the rewettable asymmetric membrane.Some exposure times can range from less than a second at high irradiance(>1 W/cm²) to several seconds or longer at a low irradiance (<50mW/cm²). The total UV energy exposure to the porous substrate can bedetermined by the UV source irradiance and the exposure time. Forexample, an array of fluorescent or germicidal bulbs can be used toexpose the porous substrate to UV radiation. The total UV energy withinthe spectral range associated with the peak lamp output can be fromabout 100 mJ/cm² to more than about 4,000 mJ/cm², from about 200 mJ/cm²to about 3,000 mJ/cm², from about 300 mJ/cm² to about 2,500 mJ/cm², orfrom about 400 mJ/cm² to about 2,000 mJ/cm².

The rewettable asymmetric membrane of the present disclosure can beprepared such that a gradient concentration of polymerized materialextends from the first major surface through at least a portion of thethickness of the porous substrate to the second major surface. Uponexposure to the ultraviolet radiation source, the photoinitiatorresiding at the first major surface can be exposed to a greater peakirradiance of UV radiation. The higher peak irradiance at the firstmajor surface can result in a higher rate of initiation at the firstmajor surface and within the pores at or near the first major surface.As the irradiance travels into the thickness of the porous substrate,the peak irradiance decreases, thus reducing the amount ofphotoinitiator decomposition and hence, polymerization within the poresof the substrate.

A gradient concentration of polymerized material can be formed resultingfrom inner filter effects. The inner filter effects can occur whencertain wavelengths are selectively filtered out by absorbing species(e.g. photoinitiators) as the ultraviolet radiation penetrates thethickness of the porous substrate. These wavelengths are effectivelyremoved or diminished. As UV radiation penetrates further into orthrough the porous substrate, the wavelength distribution of theradiation impinging on the surface can be changed resulting from theabsorption of certain wavelengths. At greater depths within the poroussubstrate, insufficient ultraviolet radiation of the prescribedwavelength region can be available to efficiently excite thephotoinitiator. The extent of polymerization of the polymerizablecomposition can decrease rapidly, forming a gradient concentration ofpolymerized material within the thickness of the porous substrate.

The sharpness of the gradient concentration of polymerized material canbe determined by the absorbance of the porous substrate at thewavelengths of the incident UV radiation. When sources other thansubstantially monochromatic sources are utilized, the absorbance isuncertain because absorbance is wavelength dependent. However, whensubstantially monochromatic sources are used, the absorbance(Beer-Lambert Law and measured using a UV-Visible spectrophotometer) atthe peak wavelength of the radiation source through a film of thepolymerizable composition at a thickness comparable to the membranethickness should be greater than 0.3, greater than 0.4, greater than 0.5or greater than 0.6. In some embodiments, the absorbance can be greaterthan 1.0 or even greater than 2.0 and as high as 10 or even 20.

The coated porous substrate selected for exposure to the ultravioletradiation source can have a thickness greater than about 10 micrometers.In some embodiments, the thickness of the coated porous substrate can begreater than about 1,000 micrometers, or greater than about 10,000micrometers. The polymerizable composition can saturate or immerse theporous substrate sufficient for wetting at least a portion of theinterconnected pores extending through the thickness of the substrateextending from the first major surface to the second major surface.

The irradiance of ultraviolet radiation received by the coated poroussubstrate can affect the extent to which the polymerizable species arepolymerized. In some embodiments, at least 10 weight percent of thepolymerizable species can be polymerized. In other embodiments, at least20 weight percent, at least 30 weight percent, or at least 40 weightpercent of the polymerizable species can be polymerized to formpolymerized material residing within the thickness of the poroussubstrate.

The irradiance of the ultraviolet radiation delivered to the coatedporous substrate can be dependent upon, but not limited to, processingparameters including the type of ultraviolet radiation source selected,the line speed (e.g., continuous process line) used, and the distance ofthe ultraviolet radiation source to the first major surface of thecoated porous substrate. In some embodiments, the irradiance can beregulated by controlling the line speed. For example, at the irradiancedelivered to the first major surface can be greater at lower linespeeds, and the irradiance delivered to the first major surface at thefirst major surface can be reduced at faster line speeds.

The irradiance of the ultraviolet radiation source delivered to a coatedporous substrate can be dependent upon the residence time as describedabove. The extent of polymerization of the polymerizable compositionthroughout the thickness of the porous substrate can be controlled bythe irradiance and can affect the concentration of polymerized materialdistributed through the thickness of the coated porous substrate. Thepeak irradiance delivered through the thickness of the coated poroussubstrate is greater than 0 mW/cm².

In some embodiments, the irradiance at the coated porous substrate uponexposure to the ultraviolet radiation source can be at least about 0.5micrometer extending into the thickness of the porous substrate from thefirst major surface. In another embodiment, the irradiance delivered tothe coated porous substrate to polymerize the polymerizable compositioncan be at least about 1 micrometer from the first major surface. In someembodiments, the irradiance delivered to the coated porous substrate canaffect the polymerizable composition to at least about 2 micrometers, toat least about 5 micrometers, to at least about 10 micrometers, or to atleast about 25 micrometers extending into the thickness of the poroussubstrate. While low irradiation and longer exposures are preferred forusing ultraviolet radiation sources, polymerizing the polymerizablecomposition as a matter of practical operation may necessitate speedsthat can require higher irradiance and shorter exposures.

FIG. 1 illustrates a cross-section of a porous substrate 5 exposed to anultraviolet radiation source 35. The membrane 5 has a first majorsurface 10, a second major surface 15, and an interstitial pore 70. Theinterstitial pore 70 can be coated with a mixture 25 comprising apolymerized material and a copolymer 20. The concentration of thepolymerized material of the mixture 25 is greater at the first majorsurface 10 than at the second major surface 15. The polymerized materialof the mixture 25 contacts the interstitial pore-mixture interface 55 inregions of the interstitial pore 70 where the polymerized material ispresent. The gradient of polymerized material is greater at the firstmajor surface 10 than at the second major surface 15. As theconcentration of the polymerized material decreases through thethickness of the porous substrate, the copolymer 20 concentration canincrease. The copolymer 20 concentration is greater such that thecopolymer 20 can collect on the surfaces of the interstitial pore 70extending through the thickness of the substrate 5 to the second majorsurface 15. A mixture-copolymer interface 60 represents a locationwithin the interstitial pore 70 where the mixture 25 contacts thecopolymer 20. At the mixture-copolymer interface 60, the concentrationof polymerized material of the mixture 25 is negligible and thecopolymer 20 coats the interstitial pore 70. A copolymer-interstitialpore interface 40, 45 exists where the copolymer 20 contacts an interiorpore volume of the interstitial pore 75 with negligible polymerizedmaterial. The copolymer and the polymerized material of the mixture 25can coat the first major surface (not shown) after exposure to theultraviolet radiation source 35. The copolymer 20 can collect or coat onthe second major surface 15 which is substantially free of thepolymerized material.

In some embodiments, a rewettable asymmetric membrane can be formedusing a multilayer structure wherein the porous substrate is coated witha polymerizable composition as previously described to provide a coatedporous substrate. A first layer can be positioned adjacent to the firstmajor surface of the coated porous substrate, and a second layer can bepositioned adjacent to the second major surface of the coated poroussubstrate to thereby form a multilayer structure. The first layer andthe second layer may be discrete pieces of materials or they maycomprise continuous sheets of materials. On a continuous process line,for example, the first layer and the second layer may be unwound fromrolls and brought into contact with the coated porous substrate. In theforegoing embodiments wherein the coated porous substrate is positioned(i.e., sandwiched) between a first layer and a second layer to form amultilayer structure, a single roller or multiple rollers may be used tometer or remove excess polymerizable composition and entrapped airbubbles from the coated porous substrate. The first layer and the secondlayer of the multilayer structure may comprise any inert material thatis capable of providing temporary protection to the membrane fromexposure to oxygen upon exiting the ultraviolet radiation source.Suitable materials for the first layer and the second layer include, forexample, sheet materials selected from polyethylene terephthalate (PET),biaxially oriented polypropylene (BOPP), fluorinate polyolefin availablefrom 3M Company and Dupont, other aromatic polymer film materials, andany other non-reactive polymer film material. The first layer should besubstantially transparent to the peak emission wavelength of theultraviolet radiation source selected. Once assembled, the multilayerstructure typically proceeds to irradiation by the ultraviolet radiationsource. After irradiation, the first layer and the second layer can beremoved (i.e., eliminated) from the multilayer structure to provide therewettable asymmetric membrane.

The thickness of the first layer of the multilayer structure cangenerally be in a range of 10 micrometers to 250 micrometers, 20micrometers to 200 micrometers, 25 micrometers to 175 micrometers, or 25micrometers to 150 micrometers. The second layer may have the same or adifferent thickness than that of the first layer. The first layer may bethe same material or a different material that that used for the secondlayer.

In some embodiments, a first layer is positioned adjacent to the firstmajor surface on the coated porous substrate to form a bi-layerstructure. The first layer can be positioned between the ultravioletradiation source and the coated porous substrate. After irradiation bythe ultraviolet radiation source, the first layer can be removed (i.e.,eliminated) from the bi-layer structure to provide the rewettableasymmetric membrane.

In another embodiment, the coated porous substrate is free of a firstlayer and a second layer. The coated porous substrate may be subjectedto an inert atmosphere (e.g., nitrogen, argon) to reduce the penetrationof oxygen (e.g., provide an oxygen free environment) to the coatedporous substrate.

The penetration of the ultraviolet radiation source can be limited bythe selection of the ultraviolet radiation source through the coatedporous substrate to produce a gradient of polymerized material withinthe rewettable asymmetric membrane that can result in differentpolymerized material compositions on the first major surface and thesecond major surface. In some embodiments, polymerized material and acopolymer can reside on the first major surface and within a portion ofthe thickness of the porous substrate. The polymerized material and thecopolymer residing within the thickness of the porous substrate can havea gradient concentration of polymerized material extending from thefirst major surface to the second major surface. The copolymer can coatthe remainder of the interstitial pores such that the second majorsurface is substantially free of the polymerized material. In oneembodiment, a rewettable asymmetric membrane has a hydrophilic firstmajor surface and a hydrophobic second major surface.

A rewettable symmetric membrane formed by the method of the presentdisclosure can have a variety of surface properties and structuralcharacteristics depending on a number of factors. These factors includewithout limitation the physical and chemical properties of the poroussubstrate, the geometry of the pores of the porous substrate (i.e.,symmetric or asymmetric), the method of forming the porous substrate,the polymeric species polymerized and retained as polymerized materialwith the surfaces (i.e., first major, interstitial pore, and secondmajor) of the coated porous substrate, optional post-polymerizationtreatments (e.g., a heating step) administered to the rewettableasymmetric membrane, and optional post-polymerization reactions (e.g.,reactions of the additional functional group of the polymerizablespecies with a compound such as a nucleophilic compound or a compoundwith an ionic group).

Rewettable asymmetric membranes of the present disclosure can exhibitvarious degrees of wettability upon exposure to various polymerizablecompositions. Wettability can often be correlated to the hydrophilic orhydrophobic character of the rewettable asymmetric membrane. As usedherein, the term “instant wet” or “instant wettability” refers to thepenetration of droplets of water into a given rewettable asymmetricmembrane as soon as the water contacts the porous substrate surface,typically within less than 1 second. For example, a surface wettingenergy of about 72 dynes or larger usually results in instant wetting.As used herein, the term “no instant wet” refers to penetration ofdroplets of water into a given substrate but not as soon as the watercontacts the substrate surface. As used herein, the term “no wetting”refers to the lack of penetration of droplets of water into a givenrewettable asymmetric membrane. For example, a surface wetting energy ofabout 60 dynes or less usually results in no wetting.

Application of polymerizable compositions onto a hydrophobic poroussubstrate and treating the coated hydrophobic porous substrate toultraviolet radiation can result in a membrane having first and secondmajor surfaces having hydrophilic character. Similarly, applying apolymerizable composition onto a hydrophilic porous substrate andtreating the coated hydrophilic porous substrate to ultravioletradiation can result in a rewettable asymmetric membrane having firstand second major surfaces having hydrophilic character.

In some embodiments of the present invention, the surface properties(e.g., hydrophilic or hydrophobic) can be altered after exposure to theUV radiation source. For example, a treatment of a hydrophobic poroussubstrate with a hydrophobic polymerizable composition after irradiationcan form an asymmetric membrane having a hydrophobic surface. Themembrane can be exposed to a hot steam/vapor (autoclave) for changingthe hydrophobic surface to a hydrophilic surface forming the rewettableasymmetric membrane.

The rewettable asymmetric membrane can be chemically asymmetric. Therewettable asymmetric membrane comprises a symmetric porous substratehaving a first major surface and a second major surface, wherein themajor surfaces (e.g., being hydrophilic) can contain polymerizedmaterial retained throughout at least a portion of the thickness of theporous substrate. The rewettable asymmetric membrane can have a greaterconcentration of polymerized material at the first major surface than atthe second major surface.

The rewettable asymmetric membrane can be physically asymmetric. Forexample, the physically asymmetric porous substrate can have a greaterconcentration of the polymerized material at the first major surfacethan at the second major surface. In some embodiments, the gradient ofpolymerized material can contribute to at least partially blocking ofthe pores on at least one major surface and an increased pore sizeextending through the thickness of the porous substrate to a secondmajor surface.

In one aspect, a rewettable asymmetric membrane is formed. Therewettable asymmetric membrane comprises a porous substrate having apolymerized material and a copolymer retained within the poroussubstrate as described in FIG. 1. The polymerized material has aconcentration greater at a first major surface than at a second majorsurface. The copolymer collects on the remainder of the interstitialpores. The second major surface is substantially free of the polymerizedmaterial. In one embodiment, the rewettable asymmetric membrane is awater softening membrane.

Rewettable asymmetric membranes formed have a greater concentration ofpolymerized material at the first major surface than at the second majorsurface and copolymer collecting on the remainder of the interstitialpores of the membrane. Rewettable asymmetric membranes can findapplications in water softening, filtration, and chromatography.

The disclosure will be further clarified by the following examples whichare exemplary and not intended to the limit the scope of the disclosure.

EXAMPLES

The present disclosure is more particularly described in the followingnon-limiting examples. Unless otherwise noted, all parts, percentages,and ratios reported in the following examples are on a weight basis.

Test Procedures

Water Flux Measurements and MgCl₂ Rejection Measurements

Water flux and MgCl₂ (magnesium chloride, salt) rejection measurementsof the rewettable asymmetric membrane prepared were measured with astirred ultrafiltration cell (model 8400; Millipore Corporation,Bedford, Mass.) having an active surface area of 41.8 cm². Thetrans-membrane pressure was set at 50 psi (pounds per square inch) underpressurized nitrogen gas. Water flux was calculated based upon theamount of water passing through the membrane as a function of time,asymmetric membrane area, and the set pressure. The MgCl₂ rejection(salt rejection) was obtained from the conductivities of the permeate(C_(p)) and the feed (C_(f)) (500 ppm MgCl₂ aqueous solution) accordingto the following equation;

${R( {MgCl}_{2} )} = {( {1 - \frac{C_{p}}{C_{f}}} ) \times 100\%}$

-   -   R=percent salt rejection.        The conductivity (C_(p) and C_(f)) was measured with a        conductivity meter (VWR Digital Conductivity Bench Meter; VWR        International, West Chester, Pa.), and the mass of permeate was        measured by an electronic balance (model TE3102S; Sartorius,        Edgewood, New York). The conductivity and the mass of the        permeate data were collected as a function of time using        Winwedge 32 computer software (TAI Technologies, Philadelphia,        Pa.). Measurements were discontinued after the salt rejection        measurements started to decline after reaching a plateau. The        salt rejection was adjusted by the feed concentration at the end        of testing.        Rewettable Asymmetric Membrane Process

Rewettable symmetric membranes were prepared by a continuous process. Apolypropylene thermally induced phase separation (TIPS) membrane asdescribed in U.S. Pat. No. 4,726,989 (Mrozinski) was die-coated with apolymerizable composition to form a coated porous substrate. The coatedporous substrate was laminated between two liners in a gap-controllednip. One of the two liners (e.g., films) was laminated to the firstmajor surface and the other liner was laminated to the second majorsurface forming a multilayer structure. The biaxially orientedpolypropylene liners ((BOPP) films of 1.18 mil (30 micrometer)thickness; 3M Company, St. Paul, Minn.) had a transmittance of about78.5 percent (UVC) and 85.9 percent (UVA). The edges of the multilayerstructure (i.e. edges of the two liners) were sealed with a pressuresensitive adhesive tape (Scotch ATG Tape 926; 3M, St. Paul, Minn.). Themultilayer structure was enveloped by the BOPP liners and the excesspolymerizable composition on the coated porous substrate was minimized.The multilayer structure was irradiated with a Quantum MicrowaveMulti-Lamp UV Curing System having a 47″ long UV window (Model:Quant-23/48R, Quantum Technologies; Irvine, Calif.). The Quantum UVSystem used either UVA lamps (26169-3, UV A 365 nm Peak Lamps TL60/10R,Philips, Somerset, New Jersey) or UVC lamps (23596-0, GermicidalSterlilamp 254 nm Lamps TUV115W, Philips, Somerset, New Jersey). Theline speed was adjusted using the machine speed display. The intensityof the ultraviolet radiation source was measured by a PowerMapradiometer (EIT UV Power MAP Spectral Response, UV: A, B, C, V, Range:Low, Head S/N 1408, Body S/N 1022; Sterling, Va.) as the multilayerstructure was carried through the UV tray. The polymerizable compositionwas polymerized forming polymerized material retained within the poroussubstrate. The multilayer substrate was collected on a roll and theliners were removed. A rewettable asymmetric membrane was recovered. Therewettable asymmetric membrane was washed with distilled water prior tofurther testing.

Example 1

A polypropylene microporous TIPS membrane (bubble point porediameter=0.8 μm, thickness of about 4.5 mil (105-115 μm (micrometers))was die coated with a polymerizable composition. The polymerizablecomposition comprised (3-Acrylamidopropyl) trimethyl ammonium chloride((APTAC) 75 wt. % in water; Sigma Aldrich, St. Louis, Mo.); butyl vinylether ((BVE) 98%; Alfa Aesar, Avocado, Lancaster, England);N,N′-methylenebisacylamide (99%; Alfa Aesar, Ward Hill, Massachusetts);and 1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Irgacure 2959, Ciba Specialty Chemicals, Tarrytown, New York) in anethanol/water mixture (70/30 volume:volume ratio). The APTACconcentration was 0.48 mol/kg, the N,N′-methylenebisacylamideconcentration was 20 mole percent relative to the concentration ofAPTAC, and the Irgacure 2959 concentration was 2 mole percent relativeto the concentration of APTAC. The molar ratio of APTAC to BVE was55:45. No pretreatment was required. The polymerizable composition wasapplied to the polypropylene TIPS membrane for forming a coated poroussubstrate. The coated porous substrate was prepared as described by theRewettable Asymmetric Membrane Process prior to forming a multilayerstructure, and prior to irradiation by the UV radiation source. Themultilayer structure was conveyed by a continuous process apparatus at aline speed of about 30.5 cm/minute. The first major surface (side A) ofthe coated porous membrane was irradiated by a UVC radiation source(light intensity about 6.0 mW/cm² as measured by a PowerMap radiometer(EIT, Sterling, Va.)). The results of Example 1 are listed in Table 2.

Example 2

Example 1 was dipped in isopropanol ((IPA), 99%; Brenntag, Butler,Wisconsin) for 10 minutes, and then air dried for forming Example 2.Example 2 was mounted on a testing holder for testing water flux andsalt rejection. The membrane was not transparent after testingsuggesting that the membrane was not fully rewetted. Testing results ofExample 2 are listed in Table 2.

Example 3

The membrane of Example 2 was dipped into isopropanol (e.g.,pretreatment) to wet the membrane before transferring into a water bathfor solvent exchange thus forming Example 3. Example 3, as a fullywetted membrane, was tested for water flux and salt rejection. Theresults of Example 3 are listed in Table 2.

TABLE 2 500 ppm Pure MgCl₂ 500 ppm Water Flux Flux (kg/m²- MgCl₂Membrane Pretreatment (kg/m²-h-psi) h-psi) Rejection (%) Example 1 None1.26 1.09 86.83 Example 2 None 0.67 0.57 84.92 Example 3 Isopropanol1.20 1.08 86.22

Example 3 had similar water flux and salt rejection performance toExample 1.

Example 4

A polypropylene microporous TIPS membrane (bubble point porediameter=0.72 micrometers, thickness of about 4.3 mil (105-115micrometer)) was die coated with a polymerizable composition. Thepolymerizable composition comprised (3-Acrylamidopropyl) trimethylammonium chloride ((APTAC); an ethylene-vinyl alcohol copolymer (EVAL27, ethylene content of about 27 mole percent; Sigma Aldrich, St. Louis,Mo.); N,N′-methylenebisacylamide; and1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Irgacure 2959) in an ethanol/water mixture (70:30 volume:volume ratio).The APTAC concentration was 0.45 mol/kg, the N,N′-methylenebisacylamideconcentration was 10 mole percent relative to the concentration ofAPTAC, and the Irgacure 2959 concentration was 2 mole percent relativeto the concentration of APTAC. The porous substrate required nopretreatment. The concentration of EVAL 27 in the polymerizablecomposition was 2.5 weight percent. The polymerizable composition wasapplied to the polypropylene TIPS membrane for forming a coated poroussubstrate. The coated porous substrate was prepared as described by theRewettable Asymmetric Membrane Process prior to forming a multilayerstructure, and prior to irradiation by the UV radiation source. Themultilayer structure was conveyed by a continuous process apparatus at aline speed of about 30.5 cm/minute. The first major surface (side A) ofthe coated porous membrane was irradiated by a UVC radiation source(light intensity about 6.0 mW/cm²). The wet membrane was washed byimmersing in water for at least two hours, such that the water waschanged periodically changed (3 times). Example 4 was cut and tested forwater flux and salt rejection. The results for Example 4 are listed inTable 3.

Example 5

The membrane of Example 4 was dipped in ethanol for 10 minutes forforming Example 5. Example 5 was air dried for 12 hours at ambientconditions. Example 5 after drying had a white color. The dried membranewas immersed in water and became transparent. Example 5 was mounted onthe testing holder for water flux and salt rejection measurements. Theresults for Example 5 are listed in Table 3.

Example 6

A polypropylene microporous TIPS membrane (bubble point porediameter=0.72 micrometers, thickness of about 4.3 mil (105-115micrometer)) was die coated with a polymerizable composition. Thepolymerizable composition comprised (3-Acrylamidopropyl) trimethylammonium chloride ((APTAC); an ethylene-vinyl alcohol copolymer (EVAL27, ethylene content of about 27 mole percent;N,N′-methylenebisacylamide; and1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one(Irgacure 2959) in an ethanol/water mixture (60:40 volume:volume ratio).The APTAC concentration was 0.45 mol/kg, the N,N′-methylenebisacylamideconcentration was 10 mole percent relative to the concentration ofAPTAC, and the Irgacure 2959 concentration was 2 mole percent relativeto the concentration of APTAC. The concentration of the EVAL 27 in thepolymerizable composition was 2.5 weight percent. The porous substraterequired no pretreatment. The polymerizable composition was applied tothe polypropylene TIPS membrane for forming a coated porous substrate.The coated porous substrate was prepared as described by the RewettableAsymmetric Membrane Process prior to forming a multilayer structure, andprior to irradiation by the UV radiation source. The multilayerstructure was conveyed by a continuous process apparatus at a line speedof about 30.5 cm/minute. The first major surface (side A) of the coatedporous membrane was irradiated by a UVC radiation source (lightintensity about 6.0 mW/cm²). The wet membrane was washed by immersing inwater for at least two hours, such that the water was changedperiodically changed (3 times). Example 6 was cut and tested for waterflux and salt rejection. The results for Example 6 are listed in Table3.

Example 7

The membrane of Example 6 was dipped in ethanol for 10 minutes forforming Example 7. Example 7 was air dried for 12 hours at ambientconditions. Example 7 after drying had a white color. The dried membranewas immersed in water and became transparent. Example 7 was mounted onthe testing holder for water flux and salt rejection measurements. Theresults for Example 7 are listed in Table 3.

TABLE 3 Pure Water Flux 500 ppm MgCl₂ 500 ppm MgCl₂ Membrane(kg/m²-h-psi) Flux (kg/m²-h-psi) Rejection (%) Example 4 1.40 1.25 82.95Example 5 1.22 1.11 87.79 Example 6 1.64 1.46 80.14 Example 7 1.21 1.2386.28

Example 5 and 7 showed rewettable asymmetric membranes which were fullyrewettable.

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that thisdisclosure is not limited to the illustrative elements set forth herein.

What is claimed is:
 1. A method of forming a rewettable asymmetricmembrane comprising: providing a porous substrate having a first majorsurface, interstitial pores, and a second major surface; applying apolymerizable composition to the first major surface of the poroussubstrate to provide a coated porous substrate, wherein thepolymerizable composition wets the porous substrate through the entirethickness of the porous substrate, the polymerizable compositioncomprising i) at least one polymerizable species; ii) at least onecopolymer comprising hydrophilic and hydrophobic groups; iii) at leastone photoinitiator; and iv) a solvent; exposing the coated poroussubstrate to ultraviolet radiation comprising a peak emission wavelengthless than about 340 nm to polymerize the polymerizable composition in agradient extending from the first major surface through a portion of thethickness of the porous substrate and provide the rewettable asymmetricmembrane; and removing a portion of the solvent from the rewettableasymmetric membrane by evaporation to coat the copolymer on at least aportion of the second major surface and the interstitial poresunoccupied by polymerized material, the rewettable asymmetric membranecomprising polymerized material on the first major surface and agradient of polymerized material extending from the first major surfaceto the second major surface with copolymer within portions of theinterstitial pores unoccupied by the polymerized material, wherein thepores are partially filled with a mixture of the polymerized materialand the copolymer, each interstitial pore comprising an interfacebetween the mixture and a coating of the copolymer on the interstitialpores that extends from the interface to the second major surface, andwherein the second major surface is substantially free of thepolymerized material.
 2. The method of claim 1, further comprisingimmersing the rewettable asymmetric membrane in a liquid bath toprecipitate the copolymer onto the portion of the interstitial poresunoccupied by the polymerized material.
 3. The method of claim 1,wherein the porous substrate is microporous.
 4. The method of claim 1,wherein the porous substrate comprises a microporous, thermally-inducedphase separation membrane.
 5. The method of claim 1, wherein the poroussubstrate comprises polyolefins, polyamides, fluorinated polymers,poly(ether)sulfones, cellulosics, poly(ether)imides, polyacrylonitriles,polyvinyl chlorides, ceramics, or combinations thereof.
 6. The method ofclaim 1, wherein the porous substrate comprises polyolefins.
 7. Themethod of claim 6, wherein the polyolefins comprise polyethylene orpolypropylene.
 8. The method of claim 5, wherein the porous substratecomprises polyamides.
 9. The method of claim 8, wherein the polyamidescomprise nylon 6,6.
 10. The method of claim 1, wherein at least one ofthe polymerizable species comprises (meth)acrylamides.
 11. The method ofclaim 1, wherein at least one of the polymerizable species comprises anionic group.
 12. The method of claim 11, wherein the ionic groupcomprises a sulfonic acid or a sulfonic acid salt.
 13. The method ofclaim 11, wherein the ionic group comprises an amine or a quaternaryammonium salt.
 14. The method of claim 11, wherein the ionic groupcomprises a carboxylic acid or a carboxylic acid salt.
 15. The method ofclaim 11, wherein the ionic group comprises a phosphonic acid or aphosphonic acid salt.
 16. The method of claim 11, wherein the ionicgroup is positively charged.
 17. The method of claim 11, wherein atleast one of the polymerizable species comprises an ionic groupcomprising a quaternary ammonium salt.
 18. The method of claim 11,further comprising at least one polymerizable species comprising anonionic group.
 19. The method of claim 1, wherein the polymerizablecomposition further comprises a crosslinker.
 20. The method of claim 1,wherein the ultraviolet radiation source comprises a plurality ofmonochromatic radiation sources.
 21. The method of claim 20, where theplurality of monochromatic radiation sources comprises excimer lampsources, mercury lamp sources, light emitting diodes, laser sources, orcombinations thereof.
 22. The method of claim 1, wherein the ultravioletradiation source comprises a plurality of fluorescent radiation sources.23. The method of claim 20, wherein the ultraviolet radiation sourcecomprises monochromatic radiation sources, fluorescent radiation sourcesor combinations thereof.
 24. The method of claim 1, further comprisingpositioning the coated porous substrate between a transparent firstlayer and a second layer to form a multilayer structure, the transparentfirst layer positioned adjacent to the first major surface and thesecond layer positioned adjacent to the second major surface, thetransparent first layer nearest the ultraviolet radiation source, andwherein exposing the coated porous substrate to the ultravioletradiation source comprises exposing the multilayer structure to theultraviolet radiation.
 25. The method of claim 24, further comprisingremoving the transparent first layer and the second layer from themultilayer structure after treating the coated porous substrate with theultraviolet radiation source having a peak emission wavelength less than340 nm.
 26. A rewettable hydrophilic membrane formed by the method ofclaim
 1. 27. The method of claim 1, wherein at least one of thepolymerizable species comprises acrylates, methacrylates, styrenics,allylics, vinyl ethers, or combinations thereof.